Add web startup analyzer to material 3 demo (#2144)

This adds a tool to measure web app startup for the Material 3 demo.

Demo:
- [Example app](https://flutter-web-perf-experiments.web.app/)
- [Material
3](https://flutter-web-perf-experiments--material3-vswzldcy.web.app/)
(open console)

---------

Co-authored-by: Brett Morgan <brett.morgan@gmail.com>
Co-authored-by: Kevin Moore <kevmoo@google.com>

web/_packages/web_startup_analyzer/example/pubspec.yaml

@@ -0,0 +1,18 @@
+name: example
+description: "flutter_web_startup_analyzer example"
+publish_to: 'none' # Remove this line if you wish to publish to pub.dev
+version: 1.0.0+1
+environment:
+  sdk: '>=3.4.0-16.0.dev <4.0.0'
+dependencies:
+  flutter:
+    sdk: flutter
+  cupertino_icons: ^1.0.6
+  web_startup_analyzer:
+    path: ../
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+  flutter_lints: ^3.0.0
+flutter:
+  uses-material-design: true